End-to-end testing of applications across networks is enabled. To test the application and the infrastructure subsystems, an application server connected to the network contains an application under test. A response simulator is connected to the other end of the network. A test module is accessible to the application and to the simulator and contains a test message to be transmitted to the simulator, and an expected response message from the simulator. The application transmits each test message and compares a response message from the simulator to an expected response message. The simulator obtains the response to each test message from the test module.